Hi all, what is the correct height of the towball, please? I have a Volvo 850 and the ball can be mounted in two positions. It is currently on the lower one which puts the top of the towball at 16 ins. My wife has a Volvo V40 with a witter bar, the ball of which is 18 ins from the ground, again measured at the top. Which is correct, or is there a tolerance? Thanks. Gary